Aksis was the rising Archon within the House of Devils. It would seem as though he assumed direct command of the Devils' Splicers after they unearthed SIVA in the Plaguelands. In an attempt to become more like the Traveler (or The Great Machine, as the Fallen refer to it) he began grafting SIVA onto his own body. The Traveler used to "belong" to the Fallen on their home world, before it arrived in our solar system, so they view it as their god. The more machine-like the Splicers become, the closer to godhood they believe that they've come. By the time that we see Aksis: Perfected, he is no longer a Fallen. He is entirely a machine, which is why "Fallen damage boost" effects don't work on him. Who controls him? We know that he has completely lost his personality and desire, replaced only by drive. This could be survival instinct that has remained, or it could be defensive algorithms enforced by the SIVA itself. Throughout the creation of the technology within the Clovis Bray institute on Mars, the team was warned multiple times that the technology was not reliable, but SIVA was engineered nonetheless. The replication chamber was setup on Earth, but there was no time to utilise it as it would seem that the Collapse occurred not long after. I believe that Aksis, like the Iron Lords at the end of the campaign, had become a slave to SIVA's faulty code. Where SIVA was following its prime command "build and multiply", Aksis was following the same directive; he was accepting SIVA offerings from the Fallen, and spreading. In the same way that the Iron Lords lost control of SIVA, Aksis succumbed to the power of the technology. It was his greed that was his undoing - in the end he was only SIVA's puppet. That's how I read/see it anyway. Hope this helped!